Structured catalysts for selective reduction of nitrogen...

Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component

Reexamination Certificate

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C423S237000, C423S238000, C423S239200

Reexamination Certificate

active

06713031

ABSTRACT:

INTRODUCTION AND BACKGROUND
The present invention relates to a structured catalyst for selective reduction of nitrogen oxides found in the lean exhaust gas stream of a combustion engine by ammonia while using a compound that can be hydrolyzed to ammonia. The catalyst is preferably used to treat the exhaust gases of vehicles powered by diesel engines.
Exhaust gases from internal combustion engines, especially from diesel and lean engines (gasoline powered internal combustion engines operated with lean fuel mixtures), contain nitrogen oxides NOx in addition to the pollutants carbon monoxide CO, hydrocarbons HC and particulate matter. In the case of gasoline engines that are run with a stoichiometrically composed air/oxygen mixture a drastic reduction of the emission of pollutants is possible through the use of three-way catalysts. They are capable of reducing the nitrogen oxides present in the exhaust gas to nitrogen while simultaneously oxidizing hydrocarbons and carbon monoxide. A stoichiometrically composed exhaust gas contains approximately 0.7 vol % oxygen. In contrast, diesel engines and lean engines have up to 15 vol % oxygen in the exhaust gas. Because of this high excess oxygen, the nitrogen oxides cannot be reduced with conventional three-way catalysts.
Reducing the nitrogen oxides selectively on a suitable catalyst while adding ammonia is known for converting the nitrogen oxides in exhaust gases that contain excess oxygen. This so called SCR (selective catalytic reduction) process is described, for example, in the European Patents EP 9 376 025 B1 and EP 0 385 164 B1. To reduce the entrainment of ammonia in the vehicle this can be produced by a compound that can be hydrolyzed to ammonia, usually urea, from which ammonia can easily be obtained by hydrolysis. However, this process requires two catalysts, a hydrolysis catalyst and a reduction catalyst.
For instance, DE 40 38 054 A1 describes a method for operating an SCR catalyst to reduce nitrogen oxides that consists of spraying a urea solution onto a heated evaporator and converting it to ammonia and carbon dioxide by means of a hydrolysis catalyst. The ammonia thus obtained is used on the SCR catalyst connected next in line to reduce the nitrogen oxides contained in the exhaust gas. Oxides with solid acid properties that contain titanium dioxide, aluminum oxide, silicon dioxide or mixed phases and compounds of these together as matrix oxide are used as the hydrolysis catalyst, where the acid properties are obtained by adding oxides of pentavalent and hexavalent elements like SO
3
and WO
3
as stabilizers and to enhance activity.
DE 42 03 807 A1 proposes, for catalytic reduction of nitrogen oxides from oxygen-containing exhaust gases using urea, a device that contain a hydrolysis catalyst that consist of fine flow channels that allow partial flows through passage reversals and penetrations or slots that are directed nearly perpendicular to the main flow. This is intended to bring about uniform distribution of the urea solution and very rapid heating of the solution. In this way the urea solution can be quantitatively converted to ammonia and carbon dioxide without the formation of harmful byproducts.
Mixtures of aluminum oxide with titanium oxide, silicon oxide, zirconium dioxide and/or H zeolites in weight ratios between aluminum oxide and the other oxides from 90:10 to 10:90 are mentioned as suitable active components for the hydrolysis catalyst in DE 42 03 807 A1.
EP 0 615 777 A1 likewise describes a catalyst arrangement of a urea hydrolysis catalyst and SCR catalyst. To obtain ammonia solid urea is used instead of an aqueous urea solution. Another oxidation catalyst is arranged downstream of the SCR catalyst to prevent possible escape of ammonia by oxidizing the ammonia to water and nitrogen.
DE 44 17 238 A1 describes another device for reducing nitrogen oxide in exhaust gas in a combustion engine operated with excess air, which consists of a hydrolysis catalyst, an SCR catalyst and a connected oxidation catalyst.
The SCR catalysts known from EP 0 385 164 B1 have a similar composition to the hydrolysis catalyst. However, they contain additional components that are necessary for the selective catalytic reduction. In accordance with the said patent, the reduction catalysts contain titanium oxide and at last one oxide of tungsten, silicon, boron, aluminum, phosphorus, zirconium, barium, yttrium, lanthanum or cerium and at least one oxide of vanadium, niobium, molybdenum, iron or copper as an additional component. It is these additional components in particular that give the catalyst good reducing properties in oxygen-containing exhaust gases.
It is characteristic for the SCR catalysts that they have a so-called window of activity. For a given volume of the catalyst, i.e., for a load on the catalyst at a specific space velocity, the conversion of the nitrogen oxides takes place optimally only in a limited temperature interval. For the indicated catalyst compositions and space velocities of 50,000 to 100,000 h
−1
this window lies between approximately 200 and 600° C. and thus roughly corresponds to the exhaust gas temperatures of diesel vehicles. If this window of activity is to be shifted to lower temperatures, the catalyst must additionally be provided with noble metals of the platinum group of the periodic system of elements or be considerably increased in volume, which, however, is only limitedly possible for reasons of space.
A disadvantage with the known catalyst systems for converting the nitrogen oxides in the exhaust gas is that they are quite long because of the two successively connected catalysts. In addition, problems arise in a cold start because of the large thermal mass of the catalyst support, due to which the overall operating system is very slow to reach its operating temperature. The exhaust gas is additionally cooled by the feed of the urea solution. The temperature difference that exists between the hydrolysis catalyst and the reduction catalyst also has an adverse affect on the dynamic conversion behavior of the catalyst system.
An object of this invention is to devise a new catalyst system that enables a shorter length of the structure than traditional systems and moreover has considerably more favorable properties.
SUMMARY OF THE INVENTION
The above and other objects of the present invention can be achieved by a structured catalyst for selective reduction of nitrogen oxides contained in the lean exhaust gas of a combustion engine by means of ammonia using a compound that can be hydrolyzed to ammonia and that is injected into the exhaust gas stream in the form of an aerosol, where the structured catalyst is in the form of a monolithic catalyst body through which a large number of flow paths for the exhaust gas run from an inlet face to an outlet face. It is a feature of the invention that the catalyst contains a reduction catalyst for selective reduction of nitrogen oxides by ammonia and a hydrolysis catalyst for the hydrolysis of the compound that can be hydrolyzed to ammonia, where the hydrolysis catalyst is applied to the reduction catalyst in the form of a coating.
A separate hydrolysis catalyst can be omitted through the arrangement of hydrolysis catalyst and SCR reduction catalyst in direct contact with each other in accordance with the invention. A considerably reduced structure length for the overall catalyst system results from this. The advantages of the invention are, however, not limited just to this geometric factor. Rather, synergistic effects that lead to a better catalytic performance compared to separate catalysts result from the close contact between the hydrolysis catalyst and the reduction catalyst.
In the catalyst in accordance with the invention the reduction catalyst forms the catalyst support for the hydrolysis catalyst. Because of this, a separate catalyst for the hydrolysis catalyst, with its very large thermal mass, is omitted. Thus, after a cold start the catalyst in accordance with the invention reaches its operating temperature, and thus its fall catalytic p

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